If you have a 2 pipe system in Parallel have you considered the possibility of NON electric zone valves on each radiator?
The fantastic aspect about non
electric radiator valves is why heat rooms your not in?
Also you can fine tune the system better with less energy use and no need to add an extra circulator.
In case of a defective pump you can always go GRAVITY flow until a replacement is found
The attic having its own thermostatically controlled valve wont get over heated as heat rises and this could be a slight problem in an attic area.
To install a separate circulator for this one zone means your going to have to install a separate thermostat PLUS are you going to invest in a night set back type or a CHEAP energy using YO YO type?
Have you considered the extra piping from the boiler to the top floor OR would it not be more advisable to just EXTEND the existing piping?
If you did install a separate circulator for this area did you properly size the circulator to prevent excessive velocity from eroding your piping?
Is your boiler sized large enough for this extra added load?
By using non electric zone valves each radiator modulates thus assuring your not asking for heat for all these rads at the same time.
Again welcome to the Harold and Sylvan Question and answer forum.
And here I bet you thought it was easy huh? SylvanLMP lover of giving options to properly tune a system